error while executing ruby's project

i am working in a ruby on rails , i have an error while displaying data. this is the erroe : undefined methodempty?' for nil:NilClasscode source erroe :-unless @history_derogations.empty? -@history_derogations.each do |item| tr td width="110px" =item.number td width="110px" =item.au_arrival_date td width="110px"`